This invention relates to the preparation of ceramic materials or articles by the pyrolysis of preceramic polymers wherein the preceramic polymers are rendered infusible prior to pyrolysis by exposure to gaseous nitric oxide. This method is especially suited for the preparation of ceramic fibers. Ceramic fibers prepared by the method of this invention have relatively low oxygen content. Such ceramic fibers with low oxygen content possess good thermal stability at temperatures up to about 1600.degree. C.
Ceramic materials have been prepared by the pyrolysis of various preceramic polymers in the prior art. Gaul in U.S. Pat. No. 4,312,970 (issued Jan. 26, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting organochlorosilanes and disilazanes. The preceramic silazane polymers were pyrolyzed in an inert atmosphere without any separate treatment to render the silazane preceramic polymer infusible.
Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting chlorine-containing disilanes and disilazanes. Fibers prepared from such preceramic silazane polymers were given a "mild heat treatment" in air before pyrolysis but there is not teaching that such a treatment rendered the fibers infusible.
Cannady in U.S. Pat. No. 4,540,803 (issued Sept. 10, 1985) obtained ceramic materials by the pyrolysis of preceramic silazane polymers prepared by reacting trichlorosilane and disilazane. The preceramic silazane polymers were not rendered infusible prior to pyrolysis to form ceramic materials.
Bartos et al. in U.S. patent application Ser. No. 748,109, filed Jun. 24, 1985, obtained infusible preceramic silazane polymers by treatment of the preceramic silazane polymer with a steam or a steam and oxygen mixture.
Lipowitz, in U.S. patent application Ser. No. 926,168, filed Nov. 3, 1986, obtained infusible preceramic polymer by treatment of the preceramic polymer with a plasma energy source. Both vinyl-containing and vinyl-free preceramic polymers were rendered infusible by plasma treatment. High energy electrons interact with neutral gas molecules in the plasma thereby forming unique species such as metastable species, atoms, radicals, and ions. Theses unique species then interact with the preceramic polymer thereby rendering the preceramic polymer infusible.
Lutz et al., in U.S. patent application Ser. No. 905,020, filed Sept. 8, 1986, obtained infusible preceramic polymer compositions by treatment of the compositions with UV irradiation wherein the compositions contained vinyl- or allyl-containing preceramic polymers, mercapto compounds, and photoinitiators.
Bujalski et al., in copending U.S. Patent Applications entitled "A Method of Producing Silicon Carbide Preceramic Vinyl-containing Polymers" and "An Improved Method of Producing Silicon Carbide Preceramic Vinyl-containing Polymers," obtained infusible preceramic polymers by thermal treatment of vinyl-containing polysilanes or by UV irradiation of vinyl-containing polysilanes.
Yajima et al. in U.S. Pat. Nos. 4,220,600 (issued Sept. 2, 1980), 4,283,376 (issued Aug. 11, 1981), 4,342,712 (issued Aug. 3, 1982), and 4,399,232 (issued Aug. 16, 1983) disclosed the use of gamma ray or electron beam irradiation to cure preceramic modified polycarbosilane fibers prior to pyrolysis. Unmodified polycarbosilane is a polymer having a skeletal structure composed substantially of recurring carbosilane units of the formula ##STR1## The modified polycarbosilane of U.S. Pat. Nos. 4,220,600 and 4,283,376 are composed mainly of the structural units ##STR2## but which also contains some boron in the skeletal polymer chain. The modified polycarbosilanes of U.S. Pat. Nos. 4,342,712 and 4,399,232 are block copolymers composed mainly of carbosilane units with titanoxane units of the formula --(--Ti--O--)--.
What has been newly discovered is a method of rendering preceramic polymers infusible prior to pyrolysis by treatment of the preceramic polymers with gaseous nitric oxide. This method represents a significant advance in the art of preparing ceramic materials or articles, especially in the art of preparing ceramic fibers.